WASHINGTON — Republican leaders scrambled to keep rank-and-file members in line ahead of a House vote on Tuesday to kill President Trump’s declaration of a national emergency on the Mexican border, as Democrats appealed to Republicans to protect Congress’s constitutional power to control federal spending.
The House’s vote on a declaration of disapproval will force Republicans to choose between the congressional prerogative over federal spending established in the Constitution and a president determined to go around the legislative branch to secure funds for a border wall that Congress has refused to grant.
Many Republicans were clearly uneasy with the president’s action, but few were ready to declare their support for legislation overturning it.
On Monday, Senator Thom Tillis, Republican of North Carolina, became the second Republican senator to say he would support the Democratic resolution.
“There is no intellectual honesty in now turning around and arguing that there’s an imaginary asterisk attached to executive overreach — that it’s acceptable for my party but not thy party,” Mr. Tillis wrote in an opinion article published Monday in The Washington Post.
Last week, Senator Susan Collins, Republican of Maine, said she would support a resolution, barring any extraneous additions. Other Republicans were holding their fire.
“I wish it wasn’t necessary” for Mr. Trump to have declared a national emergency, said Senator John Cornyn, Republican of Texas, arguing that the issue could have been avoided if Democrats had agreed to more funding for physical border barriers.
“I think it’s a shame that what should be a nonpartisan issue has turned into obviously a very partisan issue,” he added. “I’m still considering my alternatives.”
The resolution is expected to sail through the House on Democratic votes, but significant Republican defections would give it momentum in the Senate and could raise the specter — however remote — that Congress could override Mr. Trump’s promised veto, should the resolution reach his desk.
Democrats framed Tuesday’s vote as a referendum on protecting the separation of powers and Congress’s constitutional right to determine federal spending levels — an argument that appealed to several conservatives in both chambers.
Representative Joaquin Castro, Democrat of Texas and the author of the resolution, warned Monday at a news conference that without congressional interference, Mr. Trump would “try this again on other issues,” while Speaker Nancy Pelosi of California expressed confidence that her Republican colleagues’ belief in the separation of powers would push them to support the one-page resolution.
“This isn’t about the border,” she said. “This is about the Constitution of the United States.”
But Democrats were also making a less lofty case to wavering Republicans. They circulated a list of all of the possible military construction projects in each district that could lose money shifted instead to Mr. Trump’s wall.
Several lawsuits have already been filed to challenge the merits of the declaration, but the easiest way for Congress to counter it is through the resolution of disapproval, authorized by the National Emergencies Act of 1976. Once it passes the House, the Senate is required under the law to take it up within 18 days.
On Monday, Mr. Trump trained his attention on the Senate, where only four Republican votes are needed to pass the measure, should Democrats remain united, as expected. The president warned Republicans, via Twitter, not to “fall into the Democrats ‘trap’ of Open Borders and Crime!”
Several conservative senators have expressed concern that Mr. Trump’s declaration is setting a precedent that could be used by a Democratic president determined to secure funds that Congress will not give.
Others have balked at the prospect of siphoning money away from military projects. Senator Charles E. Grassley, Republican of Iowa — who said he was “leaning no” on voting for the resolution — suggested that Congress review the power to declare national emergencies granted to the president under the National Emergencies Act.
Senator Lamar Alexander, Republican of Tennessee, called Mr. Trump’s declaration “unnecessary, unwise and inconsistent with the Constitution.” Yet he declined to say how he would vote on a resolution ending it, telling reporters, “I’m going to wait and see what the resolution says.”
In the House, top Republicans were urging their members to focus on what they say is a legitimate need for border security money and the precedent set by other presidents who have declared their own national emergencies, according to one Republican aide.
Only one House Republican, Representative Justin Amash of Michigan, has signed on to the resolution to block the declaration, scorning the idea that congressional Republicans who attacked President Barack Obama’s use of executive powers “now cry out for a king to usurp legislative powers.”
Top Republicans, including Representative Kevin McCarthy of California, the minority leader, said they were confident that there will be enough support to prevent the two-thirds majority needed to override the veto. But Representative Steve Scalise of Louisiana, the Republican whip, and other members of his vote-counting team were set to lobby for votes against the resolution during a session on Monday night.
Democrats’ argument was buoyed by two letters from more than 25 former Republican lawmakers and nearly 60 former senior national security officials, who appealed to Congress to end the national emergency declaration.
“It has always been a Republican fundamental principle that no matter how strong our policy preferences, no matter how deep our loyalties to presidents or party leaders, in order to remain a constitutional republic, we must act within the borders of the Constitution,” wrote the former members of Congress, including Senators John C. Danforth, Chuck Hagel, Olympia J. Snowe and Richard G. Lugar, who implored Republicans to protect Congress’s constitutionally mandated power of the purse.
The security officials said there is neither a “documented terrorist or national security emergency at the southern border” nor an “emergency related to violent crime.”
Mr. Trump’s assertions “are rebutted not just by the public record, but by his agencies’ own official data, documents and statements,” the officials, including Madeleine Albright, the former secretary of state, and John O. Brennan, the former C.I.A. director, said in their declaration.
“Under no plausible assessment of the evidence is there a national emergency today,” they wrote.
“This isn’t about Trump or the wall, this is beyond that,” said former Representative Mickey Edwards, Republican of Oklahoma and the author of the letter to conservative lawmakers. “This is about the constitutional obligations of Congress and how much Congress is going to surrender those duties.”
“If Congress gives up the power of deciding what money you can spend and where,” he added, “it has basically surrendered its entire constitutional obligation.”
Members of the House Appropriations Committee will hold a hearing on Wednesday to examine the effect of the declaration on military construction and readiness, and members of the House Judiciary Committee will hold a hearing on Thursday in part to examine Mr. Trump’s use of powers under the National Emergencies Act.
“If we’re going to let the president, any president, on a whim declare emergencies simply because he or she can’t get their way in Congress, we have fundamentally changed the building blocks, the strong, proud building blocks that the Founding Fathers put in place,” said Senator Chuck Schumer of New York, the Democratic leader, in a speech on the Senate floor.